Tektronix, Inc., has disclosed and demonstrated the use of plasma channels to address a liquid crystal display. For example, U.S. Pat. Nos. 4,896,149, 5,036,317, 5,077,553, 5,272,472, 5,313,423, the specifications of which are all hereby incorporated by reference, disclose such structures. This technique, which provides an active addressing matrix suitable for high-line-count displays, competes with the thin-film transistor (TFT) active matrix approach commercialized by a number of Japanese companies. Plasma addressing panels are thought to be easier than TFT panels to manufacture in large sizes. The plasma addressing technology was licensed to Sony in 1993; Sony recently announced plans to commercialize 25" plasma-addressed liquid crystal (PALC) displays under the name "Plasmatron".
The PALC display, illustrated in FIG. 1, relies on the highly non-linear electrical behavior of a relatively low pressure (10-100 Torr) gas, usually He, confined in many parallel channels. A pair of parallel electrodes 14 are deposited in each of the channels 12, and a very thin glass microsheet 16 forms the top of the channels. Channels 12 are defined by ribs 15, which are typically formed by screen printing or sand blasting. A liquid crystal layer 18 on top of the microsheet 16 is the optically active portion of the display. A cover sheet 20 with transparent conducting electrodes 22 running perpendicular to the plasma channels 12 lies on top of the liquid crystal 18. Conventional polarizers, color filters, and back lights, like those found in other liquid crystal displays, are also commonly used.
When voltages are applied to the transparent electrodes, because there is no ground plane, the voltages are divided among the liquid crystal, the microsheet, the plasma channel, and any other insulators intervening between the transparent electrode and whatever becomes the virtual ground. As a practical matter, this means that if there is no plasma in the plasma channel, the voltage drop across the liquid crystal will be negligible, and the pixels defined by the crossings of the transparent electrodes and the plasma channels will not switch. If, however, a voltage difference sufficient to ionize the gas is first applied between the pair of electrodes in a plasma channel, a plasma forms in the plasma channel so that it becomes conducting, and constitutes a ground plane. Consequently, for pixels atop this channel, the voltages will be divided between the liquid crystal and the microsheet only. This places a substantial voltage across the liquid crystal and causes the pixel to switch, thus, igniting a plasma in the channel causes the row above the channel to be selected. Because the gas in the channels is non-conducting until a well-defined threshold voltage between the electrode pair is reached, and then becomes conducting, the rows are extremely well isolated from the column voltages unless selected. This high nonlinearity allows very large numbers of rows to be addressed without loss of contrast.
The PALC display relies on the use of a thin microsheet to separate the plasma from the liquid crystal. This microsheet should be as thin as possible (e.g. 1.5-2 mils), with as high a dielectric constant as possible, to minimize the voltage drop across it. Current manufacturers plan to utilize a single, monolithic piece of microsheet for this purpose, e.g., D-263 microsheet of 30-50 .mu.m thickness manufactured by Schott. However, large sheets of microsheet are difficult to manufacture, causing the availability of large, thin microsheet to be a potential limitation on the size of the PALC displays that can be manufactured. In addition, the barrier ribs in such displays commonly have quite high aspect ratios (7 mils height to 1.5 mils wide). In the past, the channels between these ribs have been made by etching into a glass substrate or by building up walls of glass on a substrate by deposition processes such as screenprinting. The technique employing etching of the channels results in channels having rounded bottoms. Techniques employing building up material to form walls result in non-vertical side walls. Both of these conditions adversely affect light transmission through the panel.
Another type of electronic information display is the plasma emissive display. In plasma emissive displays, light is emitted to create the information image to be displayed. Typically, this involves initiating an electrical discharge in a gas mixture containing Xe. The discharge ionizes the gas atoms, releasing ultraviolet radiation which strikes a phosphor, causing the phosphor to emit visible light. Where color is desired, phosphors which emit red, blue and green light upon being struck by the ultraviolet energy are used.
Several companies have demonstrated both DC and AC plasma emissive displays and several of these displays have been sold into the market place. However, to date Fujitsu Limited is the only company that is manufacturing and selling large area (e.g. 21 inch and 42 inch diagonal) color displays. One of the difficulties encountered with the manufacture of these displays is the making of the barrier rib structures, which typically are on the order of 100-130 microns high. To date such barrier rib structures are made by building up about 10-16 silk screened layers (each layer being about 10 microns or less thick), after which the structure is fired. Holding the dimensional tolerances of the multi-step silk screening process while forming the barrier ribs and maintaining alignment with the address electrode and the three phosphor layers is very difficult and only becomes more impractical as the display sizes get larger.
It would therefore be desirable to develop a more simple and robust manufacturing process for making barrier structures for use in electronic displays, which enables the formation of more accurate barrier structures having flat, vertical walls.